The invention relates to an actuator for accomplishing the swinging motion of a swinging arm, wherein the swinging arm refers to a swinging arm carrying a load and usually accomplishing a progressive movement of a so-called walking vehicle, such as a forest machine, by means of its rotating movement. Thus, the swinging arm is part of a leg that is articulated with another structure, such as a body of a vehicle, to be pivotal with respect thereto around its rotation axis. An actuator is arranged to be effective on both sides of the rotation axis, for providing a torque at the rotation axis. The swinging arm constitutes thus the leg or the like in the vehicle, or a part of the leg. The vehicle moves when it is carried by the legs in turns. The stage when the leg or the like carries the vehicle when in contact with the ground and moves it in relation to the ground, is called in this application the supporting stage, and the stage of transferring the leg in the air from one supporting stage to another, is called the transfer stage. As a rule, there are at least four legs, but usually their number is six, and each leg generally comprises at least two swinging arms which are connected to each other pivotally, the upper one being mounted pivotally on the body of the vehicle, usually on a piece pivotal in relation to the body.
In known solutions, the swinging motion of the swinging arm is accomplished preferably with two hydraulic cylinders connected at one end to the swinging arm and at the other end to a structure, in relation to which the swinging arm is arranged to pivot. The pair of cylinders alternately lifts and descends the swinging arm in relation to the rotation axis at its end. Depending on the positions of the leg, e.g. an alternating supporting stage and transfer stage of the leg are accomplished with the movements. A good example of such a leg that can have two swinging arms is the previous Finnish Patent No. 87171 by the applicant.
It is obvious that a relatively great force is required at the supporting stage, which will further require high cylinder volumes and particularly large piston work areas. Consequently, the greatest problem with the efficiency of the actuator is that because of the large work areas, a high volume flow rate of the pressurized medium must be used at the transfer stage to retain sufficiently high motion speeds at the transfer stage. Particularly in hydraulic systems, in which the pump supplies to several actuators existing at mutually different loading situations, the supply pressure of the pump is always sufficiently high in view of the maximal load. This means that the supply pressure of the pump at the transfer stage is not in correct proportion to the pressure required by the load, the pressure difference is lost in the control valves or the like and in the flow channel system, and the pump must thus be used with an unnecessarily high power input. In the above-mentioned vehicles advancing by means of the swinging motion of the swinging arm and having several actuators requiring pressurized medium for the progressive movement, one problem is still the sufficiency of pump capacity due to great momentary needs of volume flow. The hydraulic system, for example a hydraulic fluid tank, must be dimensioned according to these high peaks of volume flow. Moreover, the pump capacity cannot be increased arbitrarily without exceeding the efficiency of the driving motor.